add import for Promise interface

Summary:
Missing import might be misleading for less skilled devs.
Closes https://github.com/facebook/react-native/pull/11550

Differential Revision: D4358540

Pulled By: hramos

fbshipit-source-id: 1dada9f2ad179a64006c556c6e08e936fcc8f231

docs/NativeModulesAndroid.md

@@ -213,6 +213,8 @@ Native modules can also fulfill a promise, which can simplify your code, especia
 Refactoring the above code to use a promise instead of callbacks looks like this:
 
 ```java
+import com.facebook.react.bridge.Promise;
+
 public class UIManagerModule extends ReactContextBaseJavaModule {
 
 ...